A. Field of the Invention
The invention relates to a hydraulic circuit for controlling an automatic transmission in an automotive vehicle, and particularly a hydraulic circuit for controlling operation of forward and reverse clutches within an automatic transmission whereby the hydraulic circuit controls selection of torque transmission state, such as a forward drive state, a reverse drive state and a neutral state of the automatic transmission.
B. Description of the Background Art
An automatic transmission of a vehicle such as a construction machine or an industrial machine has forward and reverse hydraulic clutches which are hydraulically operated for forward drive and reverse drive, respectively. These clutches are controlled by a hydraulic circuit to set a vehicle in motion a forward drive state, a reverse drive state or a neutral state, where the drive state is selected by a vehicle driver or operator.
FIG. 7 shows a known structure of such a control hydraulic circuit (i.e., hydraulic circuit for control). A control hydraulic circuit 601 has a main pressure transmission line 602 which provides fluid pressure to a forward hydraulic clutch 611 and a reverse hydraulic clutch 612 via an accumulator 691, an inching valve 692 and a forward/reverse selector valve 641, and a pilot pressure transmission line 603 which is branched from the main pressure transmission line 602 and is connected to the forward/reverse selector valve 641 via a solenoid valve 671. Each of the transmission lines 602 and 603 carries working fluid that provides hydraulic pressure to the above valves for operation thereof.
The main pressure transmission line 602 extends through the accumulator 691 and the inching valve 692 and is thereafter divided into two main pressure transmission lines 621 and 622, which are connected to different portions of the forward/reverse selector valve 641, respectively. The forward/reverse selector valve 641 is internally provided with a forward selector spool 642 and a reverse select spool 652, which are opposed to each other with a spring 643 therebetween. The main pressure transmission lines 621 and 622 are connected via the forward/reverse selector valve 641 to main pressure transmission lines 623 and 624, which are connected to the forward and reverse hydraulic clutches 611 and 612, respectively.
A pilot pressure transmission line 603 is connected to the solenoid valve 671, and then is divided into two pilot pressure transmission lines 631 and 632, which are connected to different portions of the forward/reverse selector valve 641, respectively, as shown in FIG. 6. The solenoid valve 671 is internally provided with a direction select spool 672, which can move leftward or rightward depending on a signal sent to the solenoid valve 671. The pilot pressure transmission line 631 is connected to the side in the forward/reverse selector valve 641 near the forward selector spool 642, and the pilot pressure transmission line 632 is connected to the side near the reverse select spool 652.
In this control hydraulic circuit 601, a vehicle driver specifies or selects the forward, reverse or neutral setting via a control panel or control switch (not shown), whereby an electric signal is sent to the solenoid valve 671. Thereafter, the direction select spool 672 moves to a predetermined position corresponding to the forward drive state, the reverse drive state or the neutral state so that a flow path of the working fluid applying a pilot pressure is changed.
When the forward drive state is selected, the direction select spool 672 is pulled leftward by the solenoid. Thereby, the pilot pressure transmission line 631 opens so that the pilot pressure is transmitted via the pilot pressure transmission line 631 to the forward/reverse selector valve 641. The working fluid in the pilot pressure transmission line 631 flows into the side in forward/reverse selector valve 641 near the forward selector spool 642. Thereafter, the fluid pressure of the working fluid causes the forward selector spool 642 to move rightward against resistance of the spring 643. Since a right enlarged portion of the forward selector spool 642, which has closed the main pressure transmission line 623, moves rightward, the main pressure transmission line 623 opens. Thereby, the main pressure which has been cut off at an outlet of the main pressure transmission line 621 is transmitted to the main pressure transmission line 623, and further is supplied to the forward hydraulic clutch 611 to activate the forward hydraulic clutch 611. In this manner, the forward drive selected by the vehicle driver is executed.
When the reverse drive is selected, the direction select spool 672 is pushed rightward by the solenoid 671. Thereby, the pilot pressure transmission line 632 opens so that the pilot pressure transmitted through the pilot pressure transmission line 603 is transmitted to the forward/reverse selector valve 641 via the pilot pressure transmission line 632. The working fluid in the pilot pressure transmission line 632 flows toward the reverse select spool 652 of the forward/reverse selector valve 641. Thereafter, the fluid pressure of the working fluid pushes the reverse select spool 652 leftward against the biasing force of the spring 643. Since the left enlarged portion of the reverse select spool 652 which has closed the main pressure transmission line 624 moves leftward, the main pressure transmission line 624 opens. Thereby, the main pressure in the main pressure transmission line 622 which had been obstructed by the reverse select spool 652, is now transmitted into the main pressure transmission line 624, and then is supplied to the reverse hydraulic clutch 612 so that the reverse hydraulic clutch 612 is operated. In this manner, the reverse drive selected by the vehicle driver is executed.
When the neutral position is selected, the direction select spool 672 moves to a position where the left enlarged portion closes the pilot pressure transmission line 631, and the right enlarged portion closes the pilot pressure transmission line 632. Thereby, the pilot pressure is cut off by the solenoid valve 671, and is not transmitted to the forward/reverse selector valve 641. Accordingly, the forward/reverse selector valve 641 maintains the main pressure transmission lines 621 and 622 in the interrupted state, and the main pressure is transmitted to neither the forward hydraulic clutch 611 nor the reverse hydraulic clutch 612. In this manner, the neutral position selected by the vehicle driver is achieved.
However, a so-called "stick" may occur. More specifically, the select spools 642 and 652 sometimes are not able to return to the initial neutral position due to clogging of dust particles or the like which may enter the forward/reverse selector valve. In this case, the hydraulic circuit having the above structure may drive the vehicle in a direction opposite to that selected by the vehicle driver. For example, let us assume that a "stick" occurs where the forward drive has been selected in FIG. 7, and thus the forward selector spool 642 moved to the right position cannot return to the initial neutral position shown in FIG. 7. In this case, if the vehicle driver who is not aware of the stick releases the forward drive state and selects the reverse drive state, the reverse select spool 652 cannot move leftward because the forward selector spool 642 cannot return from the right position. Therefore, it is impossible to open the main pressure transmission line 624 for operating the reverse hydraulic clutch 612, and the main pressure transmission line 623 remains open so that the main pressure is supplied to the forward hydraulic clutch 611. As a result, the vehicle is driven forward although the driver has selected the reverse drive state. Conversely, the stick may occur in the reverse drive state, in which case the vehicle is reversed even if the forward drive is selected. As described above, the stick in the forward/reverse selector valve may cause a risky state that the vehicle moves in a direction opposite to the direction intended by the driver.